This invention relates to a process for producing photocatalytically active coated substrates, in particular, but not exclusively, it relates to a process for producing photocatalytically active coated glass and such coated glass.
It is known to deposit thin coatings having one or more layers, with a variety of properties, on to substrates including on to glass substrates. One property of interest is photocatalytic activity which arises by the photogeneration, in a semiconductor, of a hole-electron pair when the semiconductor is illuminated by light of a particular frequency. The hole-electron pair can be generated in sunlight and can react in humid air to form hydroxy and peroxy radicals on the surface of the semiconductor. The radicals oxidise organic grime on the surface. This property has an application in self-cleaning substrates, especially in self-cleaning glass for windows.
Titanium dioxide may be an efficient photocatalyst and may be deposited on to substrates to form a transparent coating with photocatalytic self-cleaning properties. Titanium oxide photocatalytic coatings are disclosed in EP 0 901 991 A2, WO 97/07069, WO 97/10186, WO 98/41480, in Abstract 735 of 187th Electrochemical Society Meeting (Reno, Nev., 95-1, p.1102) and in New Scientist magazine (26 Aug. 1995, p.19). In WO 98/06675 a chemical vapour deposition process is described for depositing titanium oxide coatings on hot flat glass at high deposition rate using a precursor gas mixture of titanium chloride and an organic compound as source of oxygen for formation of the titanium oxide coating.
It has been thought that relatively thick titanium oxide coatings need to be deposited to provide good photocatalytic activity. For example, in WO 98/41480 it is stated that a photocatalytically active self-cleaning coating must be sufficiently thick so as to provide an acceptable level of activity and it is preferred that such a coating is at least about 200 Å and more preferably at least about 500 Å thick (the measured thickness of the titanium oxide coatings produced in the Examples all being in the range 400 Å to 2100 Å).
However, a problem of relatively thick titanium oxide coatings is high visible light reflection and thus relatively low visible light transmission. This problem is recognised in the article in New Scientist magazine in relation to coated windscreens, where it is suggested that to reduce the effect of high reflection, dashboards might have to be coated in black velvet or some other material that does not reflect light into a coated windscreen.
EP 0 901 991A2 referred to above, relates to photocatalytic glass panes with a coating of titanium oxide of a particular crystal structure characterised by the presence of particular peaks in its X-ray diffraction pattern. The specification contemplates a range of coating thickness (with the specific Examples all having thickness in the range 20 nm to 135 nm, the thinner coatings being less photocatalytically active than the thicker coatings). The specification also contemplates a range of deposition temperatures from as low as 300° C. to as high as 750° C., but prefers temperatures in the range 400° C. to 600° C., and in all the specific Examples of the invention the titanium dioxide layer is deposited at a temperature in or below this preferred range.
The applicants have now found that by depositing the titanium oxide coatings at higher temperatures, especially temperatures above 600° C., they are able to achieve coatings with an enhanced photocatalytic activity for a given thickness, enabling the same photocatalytic performance to be achieved with thinner coatings. Such thinner coatings tend to have, advantageously, lower visible light reflection and, apparently in consequence of their higher deposition temperature, improved durability, especially to abrasion and temperature cycling in a humid atmosphere.